Screws are used to hold together various materials and generally depend upon wedging the threads and shaft of the screw into the substrate into which the screw is driven for a secure connection. However, this wedging action applies forces and resulting stresses on substrate that in certain circumstances speed substrate degradation at or adjacent to the screw and substrate interface and this in turn may lead to the loosening of the screw.
An example of this effect occurs in orthopedic screws that are driven into bones. These screws can damage the bone cells at and near the bone and screw interface, causing the bone to slowly recede from the screw and eventually causing the screw to loosen.
Another example occurs in wood where the wedging forces crushes the wood adjacent to the wood and screw interface. Also stone and concrete often crack due to the wedging forces of normal screws and bolts, especially where they are located near edges. In metal fabrication where the wedging action of a screw being driven into a metal substrate can alter the metallurgy of the substrate immediately adjacent to the screw and substrate interface. This metallurgical alteration can in certain circumstances speed corrosion and lead eventually to fastener failure.
In many cases there is a very limited elastic range in the substrate into which the screw is driven. Any recession of the substrate from the original substrate and screw interface will result in a dramatic reduction in the wedging force that the screw depends upon for purchase and hold, since the reaction forces exerted by the rigid substrate fall off rapidly with substrate recession.
One method presently used to increase the holding power of the screw is to attach a spring washer beneath the head of the screw that increases the friction and wedging forces between the thread of the screw and the substrate into which the screw is driven. The difficulty with this approach is that the spring washer pulls the screw out of the lumen into which it is driven rather than pushing it in. If the screw does become loose for any reason, this pulling force may act to accelerate the further loosing of the screw. A further difficulty with this method is that it often exerts uneven pressure on different parts of thread of the screw. This uneven application of pressure can result in lower overall friction available to keep the screw secure in the substrate and also increase in some cases the damage to the substrate that in turn may cause further loosing.
What is needed is a screw system that does not depend principally upon the wedging forces to maintain its purchase and hold in the substrate.
What is also needed is a screw system that tends to push the screw into the substrate into which it is driven rather than pull it out.
What is also needed is a screw system that evenly distributes the frictional forces along the interface between the screw and the threat on which it slides.
The present invention is a screw system that maintains purchase and hold in the substrate by maintaining a relatively low and constant force normal to the longitudinal axis of the screw even if and as the substrate recedes away from the screw and substrate interface. This force normal to the said longitudinal axis of the screw can be kept relatively constant and can be established in advance for various specific purposes to prevent unnecessary damage to the substrate into which it is inserted. Also the screw system expands as the substrate recedes maintaining intimate contact between the two. Additionally the screw system may apply a controlled and relatively constant force parallel to the longitudinal axis of the screw, pushing the screw into the hole into which it is driven and increasing the friction between the screw and the thread into which it driven thereby reducing the chance of the screw turning back out the hole.
A preferred embodiment of the invention is a system comprised of a screw and a coil or helicoil (the terms having the same meaning in this patent). Both the screw and the coil are inserted into the substrate. The helicoil is a coil formed usually from metal wire, but can be formed from any material that can have springiness imparted to it, including plastic and biodegradable plastic. The screw threads into the center of the coil, the turns of which, on the interior surface, describe the thread that meshes with the threads of the screw. The outside of the coil also forms a thread which can in turn be threaded into a thread tapped or cut into the interior walls of a hole in the substrate into which the screw and coil system are driven. In the event that the walls of the hole are not prepared with a thread, the coil may still be screwed into the hole and the coil itself will press or cut threads which will mesh with those threads (formed by the turns of the coil) on the outside of the coil.
This combination of screw and coil are well known to the art. U.S. Pat. No. 4,712,955 by Reece et al. describes a screw and helicoil system where the screw is larger than the helicoil and forces the helicoil out normal to the longitudinal axis of the coil and screw, utilizing ramp-like threads on the screw and receivers on the inside of the coil. This action creates very strong wedging forces that hold the screw and coil assembly in the hole of the substrate. This method while suitable for some purposes is not suitable where the substrate is likely to recede away from the original interface between the coil and the substrate. As explained above, even thought the wedging forces are very high, they are maintained over a small distance normal to the longitudinal axis of the coil and screw, and drop off dramatically when the substrate recedes away from the said interface.
The preferred embodiment of the invention includes a coil and screw system, but the coil, by various means tends to expand once it is inserted into a substrate independently, and not by being forced to do so by another element of the attachment system. These means are referred to herein as "expansion means". While expansion is desirable, it must not be at the expense of the loosing of the screw from the substrate as the coil moves away from the screw in response to the coil following the recession of the substrate. This patent provides those means to ensure that the connection between screw and coil is maintained under these conditions.
Unlike the wedging action of the conventional screw or conventional screw and coil combination, the coil will expand radially a relatively large distance following any recession of the substrate away from the original substrate coil interface and especially if superlastic shape memory alloy (SMA) material is used for the coil the forces exerted by the expanding coil on the interfacing substrate into which the screw and coil are driven will be relatively even, predictable and repeatable. The expanding screw and coil combination will also by various means described below maintain the purchase and hold of the screw and coil combination on the lumen of the substrate into which the said combination are driven.
A preferred embodiment of the invention includes means that tend to increase the friction between the coil and the screw and at the same time draw the screw further into the lumen of the substrate into which the screw and coil are inserted. These means are referred to herein as "torquing means" and "corrugating means".
Preferred embodiments of the invention may incorporate either those features that tend to expand the coil, once in the substrate; or those features that tend to increase friction between the coil and the screw and draw the screw into the said lumen; or both.
Expansion means both involve wrapping the coil around the screw and then introducing new conditions that allow it to expand. The first such means is to start with a coil, having springiness, that in its unloaded state has a lumen diameter larger than the outside diameter of the screw around which it will be threaded. When the coil is tightly wound around the screw, the turns of the coil falling between the threads of the screw so that they mesh, the coil will assume a more compact diameter, and if restrained by some means in this compact form will expand when the restraining means are later removed. The springy material would include conventional spring metal plastic or superlastic material, the latter of which is shape memory alloy (SMA) material that is above its austenite finish temperature in both its compressed and expanded form.
The second means to effect the expansion of this coil is to use a coil that is made of shape memory alloy (SMA) material that has been shaped set at high temperature to form a coil that has a lumen diameter larger than the outside diameter of the screw around which it will be wound. When the coil is cooled below its martensitic finish temperature, that is it is pliable, it is wound tightly around the outside diameter of the screw, the turns of the coils falling between the threads of the screw so that they mesh. The coil will then assume a more compact diameter than it's heat set size before cooling. When the coil is then heated to a temperature equal to or above its austenite finish temperature the high temperature or larger diameter shape will be recovered and the coil will expand.
Torquing means can similarly by imparted into the coil. Torquing occurs where twist is imparted to the wire along the longitudinal wire axis that forms the coil, or in the case of a coil fabricated from a tube, where twist is imparted to the tubular member that forms the coil, along its longitudinal tube axis. Torquing can occur by simply compressing or pulling the coil spring along the longitudinal axis of the gross coil (as distinguished from the longitudinal axis of the member forming the coil). Torquing may also occur in a more direct manner by grasping and twisting part or parts of the coil spring along the axis that runs longitudinally through the wire or tubular member that forms the coil. This special way of imposing torque on the spring is referred to in this patent as "tilting".
For example, the coil may be made of a wire with a rectangular cross-section, rather than the customary round one, the short sides forming the outside and inside surfaces of the coil and the long sides forming the facing surfaces between the turns of the coil. Such a coil would look like a Slinky-Toy.TM.. These turns, of the Slinky-Toy for example, are flat having the longitudinal axis of their rectangular cross-sections approximately normal to the longitudinal axis of the gross coil. Now if the said cross-sections of these turns, unlike a Slinky-Toy.TM., were angled in their unloaded state such that their longitudinal axes formed a 45 degree angle with respect to the plane passing normal through the longitudinal axis of the gross coil, each turn of the coil (if it consisted of a solid loop) would form a coned-disk shape. This shape not coincidentally would be similar to that of a belleville washer. If the coil was made of springy material, each turn of the coil would act like a belleville washer if force was applied to move the turns, or tilt their cross-sections from their unloaded angled, coned-disk configuration to their loaded flat configuration (or some configuration having a different angle than the unloaded configuration). Any coil can be tilted, but most convenient are those with cross-sections that provide points of purchase such as a rectangle, diamond, cam or triangle.
Tilting (the special way of applying torque as described above) means can similar to expanding means be effected by two physically related routes. The first route is to tilt coil turns of a springy material as described in the immediately preceding paragraph and then detachably attach the turns of the coil along all or part of its surface of the screw, such that when the constraints of the attachment are removed, the turns of the coil so treated will unload and spring back tending to project the screw further into the substrate into which it is driven by reacting against the threads of the lumen of the substrate on one hand, and on the threads of the screw on the other. The springy material that comprises the coil can include conventional spring metal, plastic or superlastic material, the latter of which is shape memory alloy (SMA) material that is above its austenite finish temperature in both its compressed and expanded form.
The second route is to make the coils from shape memory alloy (SMA) material and impart at high temperature, typically in the range of 400-500.degree. C., for nitinol SMA for example, a cross-sectional shape, that will be recovered after the coil is cooled to a temperature equal to or below the martensite finish temperature and then heated to a temperature equal to or above its austenite finish temperature. The shape so imparted will be such that when it is constrained below the martensitic start temperature and then subsequently heated to or above the austenitic finish temperature it will have tilt imparted into the turns of the coil. Since the SMA material has been heated to or above the austenite finish temperature it will be superlastic and be springy and therefore be able to spring back and as in the preceding example tending to push the screw on which it is turned forward into the substrate. If the high temperature cross-sectional shape is the same as the previous example, that is, the longitudinal axis of the coil cross-section in its unloaded state is angled 45 degrees with respect to the plane passing normal through the longitudinal axis of the gross coil, and while below the martensitic finish temperature is flattened so that the same longitudinal axis of the coil cross-section is parallel to the plane passing normal through the longitudinal axis of the gross coil; and if the coil is then constrained to maintain the said flattened cross-section, when the coil is heated to or above its austenite finish temperature it will become tilted. Once tilted the coil being now superlastic and springy may spring back and as in the previous example tend to push the screw forward into the substrate.
The turns of the coil can also be corrugated rather than tilted. The coil would meander back and forth so that the ribs of the corrugations would begin at the inside lumen of the coil and radiate or proceed out to the outside surface of the coil, usually normal to the longitudinal axis of the coil, but other angles or curves could be used in some preferred embodiments as described in the detailed description of the drawings below. This type of treatment would in some preferred embodiments of the invention be applied to the Slinky-Toy.TM. type of coil, that is one with an approximate square or rectangular cross-section. This treatment could be in addition to or instead of the tilting means elsewhere referred to herein. The means for accomplishing this would be similar to those used to effect the expanding means above. The corrugated coil could be made of springy material and in its unloaded condition could be flattened and restrained by detachable attachment on to the screw. When unloaded the coil would unload into its corrugated form and spring back tending to project the screw further into the hole into which it is driven by reacting against the threads of the lumen of the substrate on one hand, and on the threads of the screw on the other. This springy material would include conventional spring metal plastic or superlastic material, the latter of which is shape memory alloy (SMA) material that is at or above its authentic finish temperature in both its compressed and expanded form. Similarly the material could be made of shape memory metal (SMA) material, this method making use of the shape recovery regime. The recovered shape could be corrugated. The corrugated coil could be flattened into its uncorrugated form at a temperature equal to or below the martensite finish temperature and attached to the screw. When heated to or above its austenitic finish temperature and constrained into its flattened shape, the corrugated shape would be recovered, loaded and be superlastic and when no longer constrained would spring back tending to project the screw further into the hole into which it is driven by reacting against the threads of the lumen of the substrate on one hand, and the threads of the screw on the other. Rather than corrugate the coil, The coil could of course be a hollow tube and can have a compact and expanded form effect by the same means as the corrugated coil for the same purpose of providing spring-back to move the screw further into the substrate and increasing the frictional forces maintaining the screw in position.
Finally, tilting means can be additionally applied to any or any combination of methods above noted by turning the screw inside the coil while the turns of the coil are constrained by the threads of the substrate into which the screw and coil are inserted and the forward progress of the screw is stopped by the head of the screw abutting the said substrate or the tip of the screw striking a part of the substrate that prevents the screw from advancing any further. When this occurs, the threads of the screw will tend to pull the inside of the coil, to which it interfaces, in a direction opposite to the direction the screw is driven into the hole in the substrate. When the screw is turned no more, and if the said coil is made of spring material the spring-back of the said coil will tend to project the screw further into the hole into which it is driven by reacting against the threads of the lumen of the substrate on one hand, and the threads of the screw on the other.
As the coil expands, it also unwinds, and therefore if a very long threaded section is required, it is preferable that a series of small coils be placed end to end to make up the long section desired. This will reduce the friction at the coil and substrate interface which might otherwise prevent the coil from unwinding and expanding. These small coils may be separate or detachably attached so that they separate once placed inside the lumen of the said hole in the substrate. The easiest means of making the coils detachably attach is to introduce a crack or groove at intervals along the length of the wire forming the coil, such that separation will occur when the screw is turned sufficiently causing twisting forces to be imparted to the coil.
While the coil, tang or tangless, can be inserted into the substrate using helicoil insertion devices well known to the art, the preferred embodiment loads the coil onto the screw prior to insertion into the substrate as assumed above. If the coil is inserted into the substrate using an insertion tool, it will obviously require means to detachably attach the coil similar to those utilized in the screw and coil combination described in more detail herein, therefore a preferred embodiment of this invention would include an insertion tool that would have those same features as the screw and coil combination described herein.
These restraining means referred to in this patent require in most cases a detachable attachment at one or more places along the interface between the coil and the screw that and are well known to the art and include a pressure fit, pressure welding, tack welding, adhesives, flexible adhesives and biodegradable adhesives as well as water solvent adhesives the last of which would be of particular use for orthopedics and for wood work. Plastic clips or a plastic sheath can also be used that might be biodegradable or simply peal back, split or be deformed as the coil and screw are driven into the substrate. The preferred embodiment provides means for breaking the detachable attachment when the screw imparts sufficient twisting forces onto the coil as the screw is finally tightened and the coil is distorted by the action of the screw. The tacking materials could also be biodegradable and dissolve over time in the presence of solvents such as water, or corrode by biochemical or electrochemical action. Adhesive materials or mechanical attachments between the coil and the screw will be weaker transverse to the axis of the coil wire or tube, rather than parallel to it, since the coil will more easily roll in the transverse direction in response to torquing forces, than in the parallel direction where bending forces of the coil are required. This allows for the coil to be pulled into the substrate with a relatively weak bond between the coil and the screw, but once in the substrate, only a relatively little torquing action, that causes the coil to roll on the screw, is required to break the bond.
In the case of orthopedic use the use of a coil and screw combination permits greater ease of removal of the screw without damaging the bone tissue. The screw can be turned out and the threads will slide along the turns of the coil, rather than scraping along the bone. The coil can be pulled out using helicoil removal devices well known to the art, but in most cases will be left in the bone as the coil is small and is soon engulfed by the growing bone. A preferred embodiment of the invention includes coating the outside surface of the coil to encourage bone attachment by means well known to the art, including attaching porous materials to the outside surface of the coil or treating the outside surface of the coil material so that it is more porous by acid etching or other means well known to the art. Bone attachment to the outside of the coil will retard bone recession from the attachment. The application of bone attachment materials to a simple screw is not practical as removal of the screw after bone attachment to the screw surface would severely damage the bone tissue. These features noted above allowing easy removal of the screw also would apply to all other uses.
For orthopedic use, if the coil is made from superlastic shape memory alloy (SMA) material, the system that is the subject of this invention has the additional advantage that it applies the connecting forces with relatively constant forces over significant distances even as the relative positions of the attachment system and the bones change by bone recession and growth. This feature also applies to all other uses.
The even and predicable forces exerted by a coil made of superelastic material, for example superelastic nitinol alloy, on the substrate are primarily due to the relatively consistent unloading bending forces exerted by the turns of the coil as radial forces on the substrate, as the "wound up" and constrained coil is released or sprung back after or during installation of the screw/coil assembly. All or some of the turns of a superelastic coil are preloaded primarily in bending prior to installation of screw-coil assembly, by the application of a bending moment, such that the curvature of each preloaded turn is increased relative to the curvature of the turn in the unloaded state, in which no bending moment is applied. A possible loading path during the preloading of a turn of a superelastic coil, is path ABD FIG. 22, although numerous other loading paths exist whereby loading can occur to a point between B and D, for example C. During or after installation of the screw/coil assembly, the preloaded turns will unload, thereby undergoing radial expansion, in response to substrate recession or a reduction in substrate stiffness. This unloading can occur in a number of possible ways. One possible unloading path is path DEG FIG. 22, although numerous other unloading paths to any point beyond D along path DEFGA could be taken, for example, unloading to a point F, between E and G. In the case where preloading was done to a point between B and D on FIG. 22, unloading can occur to any point along numerous sub-pathways, one such example being CGA It is also possible to facilitate partial unloading of the superelastic coil turns following preloading and prior to installation of the screw/coil assembly in the substrate lumen.
Similarly, the spring-back from the tilting or torquing of the coil turns will provide an even and predictable thrusting force that keeps the screw pushing into the substrate lumen, due to the superelastic loading and unloading torque-twist behavior of the coil turns in a manner similar to the superelastic bending moment-curvature behavior described above.
For orthopedic use, the coil can be designed to respond in the same manner as the bone itself to loads and shocks, thus reducing stress concentrations at the site of the repair. Where the screw system is used to hold two bones together, the strength of the tilting and the amount of expansion of the coil can be set for the optimum pressure required to hold the two bones together to promote bone growth and mending. This feature also applies to all other uses.
The system that is the subject of this patent is particularly well suited to environments that have large swings in temperature or where vibration is present. Space structures that have huge temperature swings would benefit from the attachment system described herein. Engines and airframes would also be able to make use of the attachment system herein describe.